1. Technical Field
The present invention relates to an image processing device that converts color image data into monochromatic image data.
2. Related Art
Hitherto, to convert color image data into monochromatic image data, a method of calculating gray values from RGB values of the color image data according to the following formula (1) to obtain monochromatic image data represented by gray values is widely used. Brightness information of an original color image is reflected in the gray values. According to the method, it is possible to obtain image data of a monochromatic image to which the brightness of the original color image is reflected. In addition, “R”, “G”, and “B” of formula (1) are a red gradation value, a green gradation value, and a blue gradation value of the color image data, and “Gray” is the gray value, that is, a concentration of monochromatic black.Gray=0.3R+0.59G+0.11B  (1)
When the color image data is converted into the monochromatic image data by the gray conversion according to formula (1), a plurality of RGB values, for example, (R, G, B)=(255, 0, 0), (0, 128, 0) are converted into equivalent gray values. As described above, two colors having difference in the color image before the conversion are represented by the same gray value in the monochromatic image, and the color of the original color image before the conversion may not be differentiated from the monochromatic image after the conversion.
In JP-A-2011-23895, a method is provided in which a color image is converted into color images of cyan, magenta, and yellow, a dither matrix of generating patterns different in the color images is applied to generate dot patterns of the colors, and the dot patterns of the color are synthesized, thereby obtaining monochromatic image data. According to the method disclosed in JP-A-2011-23895, it is possible to obtain a monochromatic image formed by superimposing the different patterns of the colors of cyan, magenta, and yellow. Accordingly, for example, in the case of the conversion into (R, G, B)=(255, 0, 0), (0, 128, 0), and the like, the colors with the equivalent values are reflected to the monochromatic image, where the difference in color in the original color image becomes shapes of different patterns. Accordingly, when the color image before the conversion is subjected to gray conversion and even when undistinguishable colors are used, it is possible to convert the image data into image data of the monochromatic image with distinguishable color difference in the original color image.
However, according to the method disclosed in JP-A-2011-23895, the image of the monochromatic image data is formed by superimposing the dot pattern each color of cyan, magenta, and yellow, and thus is more detailed than each of these dot patterns, and furthermore a great number of kinds of patterns appear in the monochromatic image. For this reason, in the monochromatic image generated by the method disclosed in JP-A-2011-23895, it is difficult for the user to differentiate the pattern shapes, and it is difficult to recognize the differences in color in the original color image.